This invention relates generally to fluid operated linear actuators, and more specifically to those particularly well suited for use in portable, hydraulic cutters, punchers or the like. Still more particularly, the invention pertains to improvements in a release valve mechanism in such linear actuators.
Portable, hydraulic cutters have been developed for cutting reinforcing steel rods or wire, metal made pipe, etc., at sites of construction. They have won extensive popularity among field workers by virtue of their portability and ease of handling. There have also been developed portable, hydraulic punchers for working on sheet steel, and benders for working on steel rods and pipe.
All such devices incorporate hydraulic drive means of one type or another. A typical hydraulic drive means of this kind is a combination of a hydraulic pump and a hydraulic linear actuator or cylinder of the single acting, spring return type, both enclosed in an essentially single, compact housing. The pump supplies a hydraulic fluid under pressure needed for the extension of the linear actuator. A cutting tool such as a blade is attached to the piston rod of the linear actuator. The cutting tool cuts a desired object as the piston is driven back and forth.
A hydraulic cutter of the above general construction is described and claimed in U.S. Pat. No. 3,733,699, and its improvement in Japanese Unexamined Utility Model Publication No. 60-66411.
As disclosed in the Japanese utility model application, the hydraulic cutter includes a release valve mechanism for automatically opening the fluid discharge port of the linear actuator upon full extension. As the fluid is discharged from its fluid chamber through the port, the linear actuator will contract under spring pressure. The proper functioning of the release valve mechanism is therefore essential for the successful operation of the cutter.
As heretofore constructed, the release valve mechanism has included a valve spool received in part in an axial hollow in the piston of the linear actuator. The valve spool is movable relative to the piston into and out of abutment against one end of the actuator for covering and uncovering the discharge port formed therein. Two springs are employed for biasing the valve spool away from the discharge port. The piston holds the valve spool closing the discharge port against the forces of the two valve springs, when the actuator is fully contracted under the force of the return spring. When the actuator is extending under fluid pressure, too, the valve spool holds the discharge port closed under the force of the pressurized fluid that has been admitted into the piston hollow and which acts endwise on the valve spool. Then, upon full extension of the actuator, one of the valve springs becomes so compressed that the valve spool travels away from the actuator end thereby opening the discharge port.
The release valve mechanism of this prior art construction is objectionable for the following reasons. Upon full extension of the actuator, and when the valve spool is about to travel away from the actuator end under the biases of the two valve springs, the fluid pressure that has been admitted into the piston hollow acts endwise on the valve spool, urging the same against the actuator end in opposition to the spring biases. Consequently, in cases where the valve springs are not strong enough, the valve spool has been rather slow in uncovering the discharge port. This drawback has so far been circumvented by making the valve springs heavy enough to overcome the fluid pressure acting on the valve spool.